Alexion Pharmaceuticals Inc. (ALXN), the drugmaker whose blood-disease treatment is among the world’s most expensive medicines, engaged Goldman Sachs Group Inc. as an adviser as it prepares for a possible takeover offer from Roche Holding AG (ROG), said people with knowledge of the matter. Swiss drugmaker Roche informally approached Cheshire, Connecticut-based Alexion last week about a deal, an advance that was rebuffed, said one of the people, who asked not to be named because the overture was private. Alexion wants Goldman Sachs to help negotiate if Roche offers a price high enough to interest management, or to help defend against a possible hostile bid, another person said.